A wound rotor type induction generator is used in a power generation system having a variable power generator speed, e.g., a hydroelectric power generation system. The rotational speed of this wound rotor type induction generator is controlled by controlling the excitation frequency from the rotor side as the secondary side of the generator. To obtain a variable-frequency/variable-voltage-amplitude output from the generator, a semiconductor power converter is recently used as a secondary excitation power supply in most cases.
A conventional semiconductor power converter for a secondary excitation power supply is required to have a high power and high voltage. Therefore, it is necessary to combine a plurality of power converters in series or parallel, and use a special reactor or transformer for connection.
To obviate the need for a special component like this, there is a circuit system having a full-bridge configuration using three single-phase-output NPC (Neutral-Point-Clamped) inverters (DC→AC power converters) as semiconductor power converters.
The output power from the single-phase-output NPC inverter described above pulsates at a period twice that of the output-voltage frequency. Under this influence, a DC link voltage as the voltage of a DC link capacitor between the NPC inverter and a converter fluctuates at the same period.
Since the output frequency of the secondary excitation power supply is at most about a few Hz, i.e., very low, the fluctuating voltage width increases. To avoid this large fluctuation, the capacity of the DC link capacitor can be increased. However, if the capacity of the DC link capacitor is increased, the size and cost of the apparatus unavoidably increase.